Method for Enabling AC Coupling of High-Speed Burst Data Signals Transmitted in Optical Networks

ABSTRACT

A method for enabling alternating current (AC) coupling of high-speed burst data signals transmitted by an optical network unit (ONU). The method comprises generating a first data pattern to be sent to an optical transceiver through an AC coupling circuit, wherein the first data pattern is a direct current (DC) balanced pattern; generating a second data pattern to be sent to the optical transceiver through the AC coupling circuit, wherein the second data pattern is output prior to transmission of a high-speed burst data signal; and generating a third data pattern to be sent to the optical transceiver through the AC coupling circuit, wherein the third data pattern is output posterior to the transmission of the high-speed burst data signal.

TECHNICAL FIELD

The present invention relates generally to transmission of high-speedburst data signals in optical networks.

BACKGROUND OF THE INVENTION

Many communication networks that provide high bit-rate transport over ashared medium are characterized by non-continuous or burst datatransmission. A typical PON includes a plurality of optical networkunits (ONUs) connected to an optical line terminal (OLT) via a passiveoptical splitter. Traffic data transmission is performed over twooptical wavelengths, one for the downstream direction and another forthe upstream direction. Thus, the OLT continuously transmits downstreamdata to the ONUs and receives upstream burst data sent to the OLT fromONUs.

An ONU transmits data to the OLT during different time slots allocatedby the OLT. Transmission from an ONU to the OLT is in the form of aburst. An ONU includes an optical transceiver that receives continuousdata and transmits burst data. Every burst data transmission is precededby a Start-of-Burst (SoB) signal and followed by an End-of-Burst (EoB)signal that respectively enables and disables the optical transmitter.The OLT must identify these signals to properly recover the transmitteddata. Thus, no other signals should be output when the ONU transceiverdoes not transmit.

To allow this, PON communication standards such as the Gigabit PON(GPON) requires that a low-logic value (‘0’) signal will be transmittedwhen there is no data to output. For example, as shown in FIG. 1, fromT0 to T2 a low-logic value signal is transmitted and at T1 the SoB/EoBsignal is asserted. As illustrated in FIG. 1, the SoB/EoB signal isasserted prior to the transmission of the data to enable the laser ofthe optical transmitter to reach its working point. During the timeinterval from T2 to T3, a burst data signal is transmitted, andthereafter a low-logic value signal is transmitted. At T4, the SoB/EoBsignal is de-asserted to disable the optical transmitter.

FIG. 2 shows a schematic diagram of an ONU 200 that includes a mediumaccess control (MAC) module 210 that generates the burst data andlow-logic value signals. The MAC module 210 is a logic componentimplemented as an integrated circuit (IC). The ONU also includes anoptical transceiver 220 which its transmitter part generates andtransmits optical signals respective of the input data signals providedby the MAC module 210.

The MAC module 210 and optical transceiver 220 operate at differentdirect current (DC) levels V1 and V2 respectively. Typically, the DClevel of the MAC module 210 (V1) is significantly lower than the DClevel of the optical transceiver 220 (V2), in particular, when the sizeof the IC including the MAC module 210 is designed to support advancedsemiconductor fabrication techniques.

Typically, burst data signals generated by the MAC module 210 are offsetby a certain biased DC level. To remove the biased DC level of a burstdata signal, which is an alternating current (AC) signal, an AC couplingcircuit 230 is utilized to interface between the MAC module 210 and theoptical transceiver 220. The AC coupling circuit 230 is comprised 01serial capacitors and resistors connected in the data path between theMAC module 210 and the optical transceiver 220. The AC coupling isrequired to filter and block DC and low frequency signals. However,during AC coupling, use of coupling capacitors may cause base linewander problems to occur when a long string of information is repeatedlyincluded in a sequence of identical bits. For example, in high-speedcommunication standards, such as GPON, the number of identical bits istypically large, thus transmitting the pattern depicted in FIG. 1 wouldresults in losing information, in practical operation, when, the laserof the optical transceiver is turned on or shut down.

Some techniques discussed in the related art suggest decoding thetransmitted data using DC balanced codes, such as a Manchester code, anANSI Fiber-Channel 8B10B code, and the like. However, advancedhigh-speed communication standards, such as the GPON and 10XPON do notallow decoding the transmitted data, but rather require that thetransmitted burst data signals will be scrambled using a polynomialmethod. In addition, such techniques result in loss of data during thebeginning of the burst.

Therefore, it would be advantageous to provide a solution that limitsthe disadvantages of existing AC coupling techniques.

SUMMARY OF THE INVENTION

Certain embodiments of the invention include a method for enablingalternating current (AC) coupling of high-speed burst data signalstransmitted by an optical network unit (ONU). The method comprisesgenerating a first data pattern to be sent to an optical transceiverthrough an AC coupling circuit, wherein the first data pattern is adirect current (DC) balanced pattern; generating a second data patternto be sent to the optical transceiver through the AC coupling circuit,wherein the second data pattern is output prior to transmission, of ahigh-speed burst data signal; and generating a third data pattern to besent to the optical transceiver through the AC coupling circuit, whereinthe third data pattern is output posterior to the transmission of thehigh-speed burst data signal.

Certain embodiments of the invention also include an optical networkunit (ONU). The ONU comprises a medium access control (MAC) module; analternating current (AC) coupling circuit coupled to, the MAC module; anoptical transceiver coupled to the AC coupling circuit, wherein the MACmodule is adapted to enable AC coupling when transmitting high-speedburst data signals by generating a first data pattern to be sent to theoptical transceiver through the AC coupling circuit, wherein the firstdata pattern is a direct current (DC) balanced pattern; and generating asecond data pattern to be sent to the optical transceiver through the ACcoupling circuit, wherein the second data pattern is output prior totransmission of a high-speed burst data signal; generating a third datapattern to be sent to the optical transceiver through the AC couplingcircuit, wherein the third data pattern is output posterior to thetransmission of the high-speed burst data signal.

Certain embodiments of the invention further include an apparatus forgenerating a transmission data pattern enabling AC coupling ofhigh-speed burst data signals transmitted by an optical network unit(ONU) of a passive optical network (PON). The transmission data patterncomprises a first data pattern, wherein the first data pattern is adirect current (DC) balanced pattern; a second data pattern that followsthe first data pattern and precedes a high-speed burst data signal; ahigh-speed burst data signal; and a third data pattern that follows thehigh-speed burst data signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter that is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features andadvantages of the invention will be apparent from the following detaileddescription taken in conjunction with the accompanying drawings.

FIG. 1 illustrates a transmission pattern of an ONU.

FIG. 2 is a schematic diagram of an ONU including an AC couplingcircuit.

FIG. 3 illustrates a transmission pattern of an ONU generated inaccordance with an embodiment of the invention.

FIG. 4 illustrates a transmission pattern of an ONU to be transmitted ina GPON system generated in accordance with an embodiment of theinvention.

FIG. 5 is a flowchart describing a method for enabling efficient ACcoupling of high-speed burst signals transmitted by an ONU.

DETAILED DESCRIPTION OF THE INVENTION

It is important to note that the embodiments disclosed by the inventionare only examples of the many advantageous uses of the innovativeteachings herein. In general, statements made in the specification ofthe present application do not necessarily limit any of the variousclaimed inventions. Moreover, some statements may apply to someinventive features but not to others. In general, unless otherwiseindicated, singular elements may be in plural and vice versa with noloss of generality. In the drawings, like numerals refer to like, partsthrough several views.

Certain embodiments of the invention include a method to transmitDC—balanced signal when no data should be transmitted into the opticalline of the network. The disclosed AC coupling technique allowsunbiasing the DC level of the transmitted data signal, while maintainingthe AC swing of the signal. As a result transmission of high-speed burstdata signals is enabled without losing data. In accordance with anembodiment of the invention the high-speed burst data signals aresignals sent from ONUs to an OLT in a Gigabit PON (GPON) or higher ratePONs.

FIG. 3 shows a non-limiting diagram illustrating a transmission pattern300 generated in accordance with an embodiment of the invention. Thetransmission pattern 300 includes a first pattern (hereinafter the“fill-in pattern”) 310, second and third data pattern 320 and 330 and anoriginal burst of data 340 to be transmitted to the OLT. Thetransmission pattern 300 is generated by the MAC module (e.g., module210). The fill-in pattern 310 is a DC balanced pattern, e.g., a sequenceof alternating ‘0’ bit and ‘1’ bit, a 50% duty cycle clock, and thelike.

The second and third patterns 320 and 330 are also DC balanced patternsthat are predefined according to the requirements of the network. As anon-limiting example, if the network is a GPON, a predefined number oflow-logic value (‘0’) bits should be transmitted before and after theoriginal burst data signal 340, as required by the GPON specification,for example, GPON spec G.984.2.

FIG. 4 shows the second and third patterns 320 and 330 defined to complywith at least the GPON or the 10XPON standards. The second pattern 320includes a number of X high-logic value (‘1’) bits followed by a numberof X low-logic value (‘0’) bits (X is an integer number greater than 1).The third pattern 330 includes a number of Y low-logic value (‘0’) bitsfollowed by a number of Y high-logic value (T) bits (Y is an integernumber greater than 1). In order to ensure that the original burst ofdata 340 will not be corrupted, a number of A1 bits out of the X bitsand a number of A2 bits out of the Y bits must include low-logic value(‘0’) bits. For example, in the GPON spec G.984.2, the turn on time islimited to 16 bits, thus the value of X is 16 bits. Typically, the laserof the transceiver is ready after 12 bits, thus the SoB/EoB signal isasserted after 12 bits and A1 equals to 4 bits.

Referring back to FIG. 3, the first pattern 310 is generated and outputbetween T1 and T2, when the optical transmitter (e.g., transmitter 220)is disabled. Between T2 and T4, the second patter 320 is generated andoutput. At T3, during the second pattern, the SoB/EoB signal (not shown)is asserted to enable the optical transmitter. At the end of the secondpattern 320, the original burst data signal 340 is transmitted to theoptical line for the duration from T4 to T5.

Between T5 and T7, the third pattern 330 is generated and output. At T6during the transmission of the third patent pattern 330, the SoB/EoBsignal (not shown) is de-asserted to disable the optical transmitter. AtT7 and thereafter the first pattern 310 is output by the MAC module.

It should be noted that the burst data signals transmitted by ONUs arescrambled signals, thus having DC balance properties. The patterns 310,320 and 330 are DC balanced, thus the transmission pattern 300 has agood DC balance. Therefore, it is possible to pass the transmissionpattern 300 through an AC coupling circuit without altering burst datasignals or losing information contained in these signals. In addition,capacitors of the AC coupling circuit are chosen to have capacitancevalues that will ensure proper signal integrity regardless of thephysical characteristics of the PON.

FIG. 5 shows an exemplary and non-limiting flowchart 500 describing themethod for enabling AC coupling of high-speed burst data signals asimplemented according to an embodiment of the invention.

At S510 the fill-in (first) pattern is generated and input to theoptical transmitter.

The first pattern is a DC balanced pattern produced by a MAC module. TheMAC module is coupled to an optical transmitter using an AC couplingcircuit. At S520 a second predefined DC balanced pattern is generatedand output. The second pattern includes at least a predefined number(A1) of low-logic value (‘0’) bits. The second pattern is generated andtransmitted prior to the original burst of data signal. At S530 afteroutputting the predefined number (A1) of low-logic value (‘0’) bits (A1is an integer number greater than 1), a signal enabling the opticaltransmitter is asserted.

At S540, upon outputting the entire second pattern, the burst datasignal is transmitted. This signal is an original data signal that wasnot modified or encoded using any DC balanced codes. In certainembodiments, the burst data signal is scrambled using a polynomialfunction. At S550, upon completing, the transmission of the burst datasignal, a third predefined DC balanced pattern is generated andtransmitted. In accordance with one embodiment of the invention thethird pattern includes a predefined number (A2) of low-logic value (‘0’)bits. At S560, after outputting a predefined number (A2) of ‘0’ bits (A2is an integer number greater than 1), a signal disabling the opticaltransmitter is asserted. At S570, upon outputting the entire thirdpattern, the first pattern is generated and output. It should be notedthat all data patterns are generated by the MAC module and aretransmitted by the transceiver if the SoB/EoB signal is enabled.

The principles of the invention may be implemented as any combination ofhardware, firmware and software. Moreover, the software is preferablyimplemented as an application program tangibly embodied on a programstorage unit or computer readable medium. The application program may beuploaded to, and executed by, a machine comprising any suitablearchitecture. Preferably, the machine is implemented on a computerplatform having hardware such as one or more central processing units(“CPUs”), a memory, and input/output interfaces. The computer platformmay also include an operating system and microinstruction code. Thevarious processes and functions described herein may be either part ofthe microinstruction code or part of the application program, or anycombination thereof, which may be executed by a CPU, whether or not suchcomputer or processor is explicitly shown. In addition, various otherperipheral units may be connected to the computer platform such as an,additional data storage unit and a printing unit.

The foregoing detailed description has set forth a few of the many formsthat the invention can take. It is intended that the foregoing detaileddescription be understood as an illustration of selected forms that theinvention can take and not as a limitation to the definition of theinvention. It is only the claims, including all equivalents that areintended to define the scope of this invention.

What is claimed is:
 1. A method for enabling alternating current (AC)coupling of high-speed burst data signals transmitted by an opticalnetwork unit (ONU), comprising: outputting a first data pattern to anoptical transceiver through an AC coupling circuit, wherein the firstdata pattern is a direct current (DC) balanced pattern; outputting asecond data pattern to the optical transceiver through the AC couplingcircuit prior to transmission of a high-speed burst data signal;enabling, during the outputting of the second data pattern, transmissionfrom the optical transceiver; outputting a third data pattern to theoptical transceiver through the AC coupling circuit after thetransmission of the high-speed burst data signal; and disabling, duringthe outputting of the third data pattern, transmission from the opticaltransceiver.